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What information is needed to select a contactor?

What is a contactor?

A contactor is an electrically controlled switch used for switching an electrical circuit. The difference between contactors and relays is the contactors ability to switch higher voltage and currents. They are commonly used to control heating, lighting a motor loads. 

In order to correctly select a contactor for your application, you will first need to consider the following points.

Coil – used to energise the contactor to switch the main contacts on and off

Main Contacts (Poles) – the current carrying part of the contactor

Auxiliary contacts – used for control and signalling circuit applications


1. Coil Selection 

  • AC or DC operation
  • Control circuit voltage e.g. 24VAC, 230VAC, 400VAC, 24VDC etc. Note: If AC then select correct supply frequency, if DC is low power consumption option needed?

2. Main Pole Selection

  • Number of normally open (N/O) and/or normally closed (N/C)
  • Switching current rating (Amps)
  • AC or DC voltage

What unitisation category is required e.g. AC-1, AC-3, DC-21 etc. This is based on the type of inductive or resistive load, and the number of stop/start operating cycles

3. Auxiliary contact Selection 

  • Quantity of normally open (N/O) and/or normally closed (N/C) contacts

Note: The auxiliary contacts are not necessarily rated at the same current rating as the main poles. Also time delayed versions are available on some models 

In addition to the above, the following information should be gathered to help select the right contactor for the application.

  • Environment and ambient temperature
  • Stand alone or modular design
  • If any interlocking, latching, overload, enclosures, timers, coil surge suppressors or busbars are needed.

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3 May 2017, 13:40